pieces of vegetables are dried, the cell walls can acquire a glassy
state, leading to a hard and possibly brittle product.
Bakingof a dough is one of the most common processes. At high
temperature water can readily be removed and cooling then can lead
to a glassy state, as in a hard biscuit.
High-pressureextrusioninvolves comparable changes. The product
becomes hot and vapor bubbles are formed in it. The bubbles
expand when the material leaves the extruder, and water vapor can
readily escape. Upon cooling, a solid foam is formed, i.e., a glass
containing many air cells. This is applied in manufacturing dry
snacks.
Spray dryingis applied to liquid foods like skim milk. Drying must
proceed very fast, since otherwise lactose will crystallize. By
dispersing the milk into fine droplets that are brought into hot
air, the drying can be almost completed within a few seconds,
whereby crystallization is prevented.
Alsofreeze dryingcan be applied to solutions. Water evaporates
(desublimates) at low temperature and low pressure from the ice
crystals present, leaving a highly porous structure of dry solutes,
which often is in a glassy state.
Freeze concentrationis a very common process; it is the main subject of
Section 16.2.Texture. A hard biscuit has a crisp or brittle texture. This implies
that it deforms in a fully elastic manner upon application of a force, until it
breaks (snaps) at a relatively small deformation. Breakage goes along with a
‘‘snapping’’ sound. It appears from empirical observations that a crisp
material has an apparent viscosity of at least 10^13 or 10^14 Pa?s. The water
content or temperature above whichcrispness is lostclosely corresponds to
Tg. Sensory evaluation shows that an increase in water content by 2 or 3
percentage units, or in temperature by 10 or 20 K, can be sufficient to
change a crisp food into a soft (rubbery) material.
A comparable phenomenon is that a solid material obtained by freeze
drying can change into a highly viscous liquid upon increasing water content
or temperature; this is generally calledcollapse, since the desirable porous
structure of the freeze-dried product is lost. The collapse temperature tends
to be a few kelvins aboveTg.
If the glass consists in substantial part of low-molar-mass components,
like sugars, the material tends to become quitestickysomewhat aboveTg.
This can occur with several spray-dried powders that take up water from the
air; the stickiness then causes the powder particles to form a coherent mass,
a phenomenon calledcaking. On the other hand, powder particles can be